Single stage delignification and bleaching of chemical pulp with oxygen in the presence of a protector

ABSTRACT

Delignification and bleaching of chemical pulp is achieved by treating the pulp in a single stage with oxygen at elevated temperature and pressure in the presence of a protector selected from the group consisting of Na2S2O4 and Na2Sx, where x is an integer from 1 to 4.

United States Patent 1 u 11 3,716,448

Roymoulik Feb, 13, 1973 1 SINGLE STAGE DELIGNIFICATION [56] References Cited AND BLEACHING OF CHEMICAL I v UNITED STATES PATENTS 3,384,533 5/1968 Robert et al ..l62/65 PRESENCE OF A PROTECTOR Inventor: Sunanda K. Roymoulik, Suffern,

Appl. No.: 40,429

U.S. Cl. ..l62/65, 8/110, 8/1 1 l,

. 162/82 I t. Cl ..D2lc 9/10 Field of Search ..162/65, 82, 83; 8/1 l0, lll

FOREIGN PATENTS OR APPLICATIONS 56,526 1922 Denmark; ..l62/83 Primary Examiner-S. Leon Bashore Assistant Examinen-ArthurL. Corbin Attorney-Robert S. Sanborn, Walt T. Zielinski, Jay S. Cinamon and Howard R. Goldberg [5 7 ABSTRACT Y 8 Claims, No Drawings SINGLE STAGE DELIGNIFICATION AND BLEACI-IING OF CHEMICAL PULP WITH OXYGEN IN THE PRESENCEOF A PROTECTOR The present invention, relates to a single-stage process for the delignification and bleaching of chemicalpulps.

The commercial bleaching sequences used in modern-day pulp mills have anywhere from three to six stages. In these sequences chlorine, in one form or another, represents a major constituent. Some of the chlorine-containing compounds which are used are chlorine gas, chlorine dioxide and sodium hypochlorite. While these bleaching sequences are effective, they have certain disadvantages. A major disadvantage in the multiple-stage chlorine-containing bleaching sequences is the liberation of objectionable chlorine-containing effluent materials. Today, when intensive efforts are being made to abate pollution of our air and water, a chlorine-free bleaching sequence is most desirable. Another disadvantage is that when five or six stages are employed in a bleaching sequence, the cost of the chemicals necessarily becomes quite expensive. Another problem created by these chlorine-containing compounds is that they degrade the cellulosic portion of the pulp with consequent impairment and diminution in its physical properties.

In order to avoid the problems involved in multiplestage bleaching sequences involving the use of chlorine, Watanabe, US. Pat. No. 3,251,730, discloses treating a substantially chlorine-free pulp in aqueous alkaline solution followed by treatment with oxygen under pressure at an elevated temperature. The Watanabe patent discloses that by following his process the reduction in pulp viscosity is minimized. It has been found, however, that this oxygen-alkali sequence, like most of the other oxidative bleaching agents, is not specific. Namely, the treatment not only attacks the chromophoric groups derived from the chemically modified lignin, but also the carbohydrate portion of the molecule, i.e., cellulose and hemicellulose, resulting in degradation of the pulp, as manifested by loss in the degree of polymerization. As a result, there is a loss in pulp strength, yield and viscosity.

One approach which has been taken to obviate the above-mentioned difficulty in oxygen-alkali bleaching is to be found in Meylan, et al, US. Pat. No. 3,384,533 which discloses that the use of a catalyst, for example, magnesium carbonate, aids in avoiding excessive degradation. However, the use of magnesium carbonate introduces magnesium and carbonate ions into the water in the system. Since these ions are not found in a typical chemical pulping process, either sulfate of sulfite, they would, over a period of time, result in the formation of scales and encrustations on process equipment. Recirculation of the effluent water would, as a result, be impossible.

It is an object of the present invention to provide a process for the delignification and bleaching of chemical pulps while minimizing degradation and depolymerization of the cellulose portion of the pulp.

It is another object of the present invention to provide a process for the delignification and bleaching .of chemical pulps whicheliminates the necessity of employing chlorine-containing compounds.

The above and other objects are accomplished by contacting or reacting an unrefined, digested chemical pulp, prepared by either the sulfate or sulfite process, having a consistency of from about 3 percent to about 35 percent, by weight of the oven-dry pulp, with oxygen, at a temperature of from about C. to about 140C. and at a pressure of from about 60 p.s.i. to about 130 p.s.i. for approximately one-half hour to about eight hours in the presence of at least 0.2 percent, based on the weight of the pulp, of a protector compound selected from the group consisting of polysulfides having the formula Na S wherein x is an integer from I to 4 and Na S O The use of the term protector throughout this application shall mean either an oxidizing or a reducing agent which is capable of preventing the degradation and depolymerization of the cellulosic portion of the pulp by reacting with the carbonyl end groups to convert them into either hydroxyl groups, when a reducing agent is used, or carboxyl groups, when an oxidizing agent is used.

It is preferred that the process of the present invention be carried out at a temperature of from about C. to about 110C. and even more preferably at a temperature of from about C. to about 105C. and most preferably at C. It is preferred that the pressure during the reaction be maintained at from about 90 p.s.i. to about 1 10 p.s.i. and even more preferably at about 100 p.s.i. Naturally, however, the time of reaction, the temperature and pressure of reaction are all interdependent, as well as being dependent upon the nature of the pulp that is employed, the end use of the pulp, and the necessity for striking a compromise between the sought for delignification and the prevention of cellulose degradation.

When sodium hydrosulfite is used as the protector, since it is a reducing agent, it protects the carbonyl end groups of the cellulose against degradation by converting them to alcohol groups. When a sodium polysulfide is used as the protector, since it is an oxidizing agent, it stabilizes the cellulose against degradation by converting the carbonyl end groups to carboxyl groups. Na S is the preferred polysulfide.

The concentration of either sodium hydrosulfite or sodium polysulfide should be at least about 0.2 percent, based upon the weight of the pulp. Preferably, however, the concentration should be from about 2 percent to about l0 percent. The 10 percent figure represents an economical upper limit. Concentrations in excess of 10 percent can be used, but no additional benefits are obtained. 7

An important advantage of the process is that since the protectors contain only sodium and sulfur ions, both of which are used in conventional sulfite or sulfate pulping operations, the effluent from the process can be fed back into the chemical recovery system thereby preventing stream pollution which would otherwise occur if the effluent was emptied into the water as is conventionally practiced.

Optionally, sodium hydroxidein concentrations of from about 1 percent to about 5 percent can be employed in the present process. Sodium hydroxide can be used advantageously when pulps having high lignin contents, for example 5 percent or above, are employed, to speed up the rate of reaction, or when low concentrations of Na S Or, or Na S namely, 2 percent or less are employed. However, when the concentration of the protector is 2 percent or above, it is unnecessary to use sodium hydroxide.

The following examples illustrate the process of the present invention.

in each of the four examples listed below a 100 gram sample of a hardwood pulp, which had been prepared in accordance with the Kraft process and which had subsequently been washed with deionized water, was oxygenated in an electrically heated pressure reactor Reaction Pressure 100 psi Reaction Time hours Pulp Consistency -22% Na S. 2% on O.D. pulp (Polysulfide) NaOH 2% on O.D. pulp EXAMPLE 3 Reaction Temperature 100C Reaction Pressure (0,) 100 psi Reaction Time 2 hours Pulp Consistency 20-22% Na,S O 2.5% on O.D. pulp EXAMPLE 4 Reaction Temperature 100C Reaction Pressure (0 100 psi Reaction Time 5 hours Pulp Consistency 20-22% Na S 5% on O.D. pulp The pulps treated in accordance with Examples 1 through 4 were evaluated for Viscosity (Tappi T230 su- 66), Kappa No. (Tappi T236 m-60) and Brightness (Tappi T217 m-48) and compared against an untreated pulp. The results are listed in Table I below.

TABLE I UNTREATED T R E A T E D Ex. Ex. Ex. Ex.

1 2 3 4 Kappa No. 14.8 7.4 6.3 8.4 8.6 Viscosity 26.74 19.31 19.96 19.14 16.68 (Centipoises) Brightness 32.4 49.3 57.0 43.0 43.0

What is claimed is:

1. A process for delignifying and bleaching a chemical pulp while minimizing cellulose degradation, which comprises:

reacting said pulp in a single stage with oxygen in the presence of a protector at a concentration of from about 0.2 percent to about 10 percent based on the weight of the pulp, said protector being selected from the group consisting of Na S O and Na S wherein x is an integer from 1 to 4.

2. The process as recited in claim 1 wherein the consistency of the pulp is from about 3 percent to about 35 percent.

3. The process as recited in claim 1 wherein the reaction is carried out at a temperature of from about C. to about 140C.

4. The process as recited in claim 1 wherein the reaction is carried out at a pressure of from about 60 p.s.i. to about p.s.i.

5. The process as recited in claim 1 wherein the time of reaction is from about one-half hour to about 8 hours. y

6. The process as recited in claim 1 wherein the reaction is carried out in the presence of from about 0.5 percent to about 5 percent of sodium hydroxide, based on the weight of the pulp, and the concentration of the protector is 2 percent or less, based on the weight of the pulp.

7. The process as recited in claim 1 wherein the protector is Na s, and its concentration is from about 2 percent to about 10 percent, based on the weight of the pulp.

8. The process as recited in claim 1 wherein the protector is Na S O and its concentration is from about 2 percent to about 10 percent, based on the weight of the pulp. 

1. A process for delignifying and bleaching a chemical pulp while minimizing cellulose degradation, which comprises: reacting said pulp in a single stage with oxygen in the presence of a protector at a concentration of from about 0.2 percent to about 10 percent based on the weight of the pulp, said protector being selected from the group consisting of Na2S2O4 and Na2Sx, wherein x is an integer from 1 to
 4. 2. The process as recited in claim 1 wherein the consistency of the pulp is from about 3 percent to about 35 percent.
 3. The process as recited in claim 1 wherein the reaction is carried out at a temperature of from about 70*C. to about 140*C.
 4. The process as recited in claim 1 wherein the reaction is carried out at a pressure of from about 60 p.s.i. to about 130 p.s.i.
 5. The process as recited in claim 1 wherein the time of reaction is from about one-half hour to about 8 hours.
 6. The process as recited in claim 1 wherein the reaction is carried out in the presence of from about 0.5 percent to about 5 percent of sodium hydroxide, based on the weight of the pulp, and the concentration of the protector is 2 percent or less, based on the weight of the pulp.
 7. The process as recited in claim 1 wherein the protector is Na2S4 and its concentration is from about 2 percent to about 10 percent, based on the weight of the pulp. 